The need to reform and reuse solutions of acids continues to increase with efforts to protect the environment and to conserve resources. Thousands of tons of acids are made and used annually. These acids after use are generally neutralized with caustic and lime and the soluble salts are discharged into water resources. Prior electrodialytic processes, see U.S. Pat. No. 4,636,288, provide a satisfactory method for reforming acidic solutions containing multivalent metal salts. These processes comprise electrotransporting multivalent metal cations from an acidic solution through a cation permeable membrane and insolubilizing the metal cation. The electrotransported metal cation is replaced by hydrogen ions to form an acid of the salt anions. Unfortunately, metal cations form complexes with anions that have no electrical charge or have a negative charge and the metal cation of the complex cannot be electrotransported from the acidic solution through a cation permeable membrane and the anions formed into acids. There are also problems in the prior art processes of electrotransporting metal cations through cation permeable membranes. Multivalent cations form insoluble salts in membranes or on the surface of membranes that reduce or preclude the electrotransport of cations. There are few commercial solutions that have no calcium or other metal cations and most solutions contain cations of two or more metals and anions of two or more acids. It is an object of the instant invention to provide an electrodialytic process that does not require electrotransport of metal cations that is suitable for reforming solutions of acids which contain metal complexes and/or salts of metal cations whereby the acids can be used again and again.
Electrodialysis is a well-know art (See U.S. Pat. Nos. 4,636,288; 4,325,792; 4,439,293, the disclosures of which are hereby incorporated by reference.) Electrodialysis is the transport of ions through ion permeable membranes as the result of an electrical driving force. The process is commonly carried out in an electrodialytic cell having an anolyte compartment containing an anolyte and an anode separated by an ion permeable membrane from a catholyte compartment containing a catholyte and a cathode. The ion permeable membrane can be permeable to cations or anions. The anion permeable membrane usually has fixed positive charges and, as the name implies, is permeable to anions and relatively impermeable to cations. The cation permeable membrane usually has fixed negative charges and is permeable to cations. The electron transfer reactions at the electrodes upset the equivalence of positive and negative ions which causes current to flow through the cell. Oxidation reactions occur at the cell anode and reduction reactions occur at the cell cathode. Water is oxidized to hydrogen ions and oxygen at the cell anode and water is reduced to hydroxyl ions and hydrogen at the cell cathode.
There are many complexes of metal cations (See Inorganic Chemistry, Fritz Ephraim, Fifth Edition by R. C. L. Throne and E. R. Roberts.) The complexes can be defined as substances formed by the combination of components which are already saturated according to the classical concepts of valency. The coordination number is commonly six and the complex is not an ion, but is an electrically neutral compound. There are no known groups which definitely confer a positive charge on a complex of a metal cation. There are, however, substances which can cause an increase in the negative valency and complexes can have a negative charge.
When metals are etched, electropolished, bright-dipped or pickled with acids, such as phosphoric and hydrofluoric, a mixture of complexes and salts of metal cations are formed in the acid or mixtures of acids. The increasing need to protect the environment and to conserve resources make the reuse of the acids and metals desirable. These acidic solutions usually contain two or more acids, two or more metal cations and a mixture of complexes and salts of metal cations and anions of the acids. It is possible to partially reform the acids using electrodialysis (See U.S. Pat. No. 4,636,288.) by removing the metal cations of the salts. However, the anions associated with the metal complex are not reformed and in reuse the concentration of the metal complex increases in the mixture of acids and, at some point, the solution of acids must be replaced or the metal complexes removed. The concentrations of acids and the level of salts and complexes permissible in the acids vary widely in the many finishing processes for metals. These complexities essentially preclude partial reformation of the acids in commercial processes. It is an object of the present invention to provide a process suitable for reforming acids and mixtures of acids in aqueous solutions containing complexes of metal cations or salts of metal cations and mixtures of complexes and salts of cations of one or more metals.